1. Field of the Invention
The present invention relates to antistatic or electrically conductive thermoplastic polyurethanes and a process for making them. More particularly, the invention relates to the preparation of these thermoplastic polyurethanes by incorporation of carbon black having specific physical characteristics into a thermoplastic polyurethane while maintaining the temperature below the melting point peak of the rigid crystalline segments as determined by differential scanning calorimetry.
2. Description of the Prior Art
Thermoplastic polyurethane elastomers have been known for a long time. Their commercial utility is based on their ability to combine desirable mechanical properties with the advantages of economic thermoplastic processing. A wide range of mechanical properties can be achieved by using different starting materials. An overview of thermoplastic polyurethane elastomers, their properties and applications, is given in Kunststoffe 68 (1978), pp. 819-825, or in Kautschuk, Gummi, Kunststoffe 35 (1982), pp. 568-584.
Thermoplastic polyurethane elastomers can be produced either continuously or by batch processes. The most well known of the continuous processes, the so-called sheet process and the extrusion process, are widely used on a commercial scale.
In British Pat. No. 1,057,018, for example, a polymer is prepared from an essentially linear hydroxyl compound and excess organic diisocyanate, fed into a mixing head through a metering pump, and mixed therein with a specified amount of a low-molecular-weight diol. The resulting reaction mixture is fed onto a conveyor belt and passed through an oven heated to from 70.degree. to 130.degree. C. until it solidifies. The reaction product is then granulated and tempered at temperatures up to 120.degree. C. for from 6 to 40 hours. The resulting product can then be processed into molded parts by the usual methods, for example, by injection molding.
In the extrusion process, which is described in German Pat. No. 20 59 570 (U.S. Pat. No. 3,642,964), the starting components are fed directly into the extruder and the reaction is performed in the extruder under the specified process conditions. The resulting polyurethane elastomer is converted into a thermoplastic directly, extruded as a strand, cooled in an inert gas atmosphere until it solidifies, and is then granulated. The disadvantage of this process is that the resulting thermoplastic polyurethane elastomer is not suitable for producing sheets, fine sections, and tubing or hose. Thermoplastic polyurethane elastomers of identical composition are transparent when made by the extrusion process, but have an opaque appearance when made by the continuous sheet process. Opaque thermoplastic polyurethane elastomers can be processed into films which are not subject to blocking, while transparent thermoplastic polyurethane elastomers are not suitable for this purpose.
Polyurethane elastomers are frequently modified by incorporating suitable additives. In order to improve resistance to hydrolysis, ortho-substituted diarylcarbodiimides can be utilized in amounts of up to 2 weight percent of the total mixture. Polyurethane elastomers having low coefficients of friction are obtained when solid or liquid lubricants or mixtures thereof are blended with the elastomers. Suitable lubricants, for example, are graphite or molybdenum disulfide, which are generally mixed with paraffin or silicone oil. A maximum of 3 weight percent of such lubricants is generally added, since greater amounts do not increase the lubricating effect but do decrease mechanical properties. Improved antistatic properties can be obtained by adding conductive pigment carbon blacks or antistatic plasticizers. However, since pigment carbon blacks produce a large and undesired increase in viscosity, only a few percent can be used successfully.
Various types of fillers, similar to those used in rubber technology, can be incorporated into polyurethane elastomers in relatively large amounts. For example, increasingly greater reinforcing effects can be achieved by adding kaolin, precipitated silicic acid, and pyrogenically obtained silicic acid, respectively. Carbon black fillers can sometimes exhibit a higher level of mechanical properties than light colored fillers. Rollable polyurethane elastomers often incorporate from 60 to 100 weight parts of an inactive of semi-active filler for each 100 weight parts elastomer and from 20 to 60 weight parts of a highly active filler (Kunststoff Handbuch, vol. VII, Polyurethane, by R. Vieweg and A. Hochtlen. Munich: Carl Hanser Verlag, 1966, pp. 206 ff).
Polyurethane elastomers modified in this manner, however, only have slight conductivity. Moreover, there is a severe decrease in mechanical properties as well as increasing difficulty in processing these elastomers into molded parts.